The present invention is related to a method for substantially eliminating a fern-like pattern on metal strip which is being treated in a series of electrolytic cells to provide an electroplated metal or metal-alloy coating thereon, and particularly to a method of applying a uniform film of electrolyte solution to a surface of the strip which is being plated for at least 0.1 second immediately prior to entry of said surface into each cell.
In the electrogalvanizing of steel strip, electrolytic cells are provided in series so that the strip is passed sequentially through the cells. Electrical current in each cell flows through a zinc-containing electrolyte solution from one or more anodes to a conductor, bonding zinc or iron-zinc alloy to the strip. The cells may be of three primary types: horizontal, vertical or radial. Between the cells, deflector rolls are provided for directing the strip upwardly out of a previous cell and downwardly into a next succeeding cell. There is a tendency for the strip to carry electrolyte solution from one cell to the next, but the deflector rolls remove most of the electrolyte by contact with the strip. Because the deflector rolls are not perfectly flat, a non-uniform layer of electrolyte passes through in gaps between the roll and the strip and is carried on the strip into the next cell. As a result, for reasons which are not completely understood, a fern-like pattern appears on the strip after electroplating. The pattern appears with greatest severity on steel having a 10 to 20% iron, balance zinc coating although it has also been noted on steel strip having a pure zinc-plated coating. In the most severe form, the pattern cannot be covered by paint of the thicknesses used on automotive body parts. For this reason, the pattern is undesirable and hinders acceptance of the product for these applications.
Various approaches have been taken in attempts to eliminate the fern-like pattern. Most of these have been directed at removing the electrolyte solution from the strip prior to its entry into a next succeeding cell after it leaves a prior one. For example, it is known to immerse a lower portion of the deflector roll in a bath of water to remove electrolyte from the roll. A showing of such a bath for immersion of a roll is contained in U.S. Pat. No. 2,793,993, Stock, at 12 in FIG. 1 thereof. It is also known to spray the strip with water both prior to and after its contact with the deflector roll. In an unrelated application, a water spray is shown at the latter location in U.S. Pat. No. 3,563,863, Vierow. Finally, wringer rolls may be provided to remove as much electrolyte as possible, again either before or after the deflector roll. None of these have proved successful in preventing occurrence of the fern-like pattern.
It is also known to pretreat the strip prior to its contact with the deflector roll for various purposes. In U.S. Pat. No. 4,401,523, Avellone, a strip conditioner station 32, FIG. 1, is provided ahead of the plating section 14. At the conditioner station a zinc sulphate spray is applied to the strip to enhance plating performance by forming a non-porous barrier film for inhibiting corrosion of the pickled and cleaned steel surface prior to plating and by acting as a seed for the plating process. The provision of concentrated electrolyte ahead of the deflector roll and maintaining a steam atmosphere around the roll is shown in U.S. Pat. No. 3,796,643, Swalheim in the electroplating of tin. This reference also discloses the use of wringer rolls 24 and water sprays 4. The application of electrolyte solution by sprays located ahead of the deflector rolls is also known in various electrogalvanizing processes. In an unrelated application, a suspension of abrasive substances in an electrolyte solution is applied after the strip passes over a deflector roll as disclosed in U.S. Pat. No. 3,691,030, Strosznski in the electrochemical oxidation of the surfaces of aluminum or zinc substrates for making lithographic printing plates. And finally, U.S. Pat. No. 3,591,467, Carter, discloses the application of a protective fluid to exclude electrolyte from the surface which is not to be plated. Header apparatus similar to that used for practicing the method of the present invention is disclosed in U.S. Pat. Nos. 1,751,960 and 1,987,962.